Matthew D. Ford

PIV-Measured Versus CFD-Predicted Flow Dynamics in Anatomically Realistic Cerebral Aneurysm Models

Computational fluid dynamics (CFD) modeling of nominally patient-specific cerebral aneurysms is increasingly being used as a research tool to further understand the development, prognosis, and treatment of brain aneurysms. We have previously developed virtual angiography to indirectly validate CFD-predicted gross flow dynamics against the routinely acquired digital subtraction angiograms. Toward a more direct validation, here we compare detailed, CFD-predicted velocity fields against those measured using particle imaging velocimetry (PIV). Two anatomically realistic flow-through phantoms, one a giant internal carotid artery (ICA) aneurysm and the other a basilar artery (BA) tip aneurysm, were constructed of a clear silicone elastomer. The phantoms were placed within a computer-controlled flow loop, programed with representative flow rate waveforms. PIV images were collected on several anterior-posterior (AP) and lateral (LAT) planes. CFD simulations were then carried out using a well-validated, in-house solver, based on micro-CT reconstructions of the geometries of the flow-through phantoms and inlet/outlet boundary conditions derived from flow rates measured during the PIV experiments. PIV and CFD results from the central AP plane of the ICA aneurysm showed a large stable vortex throughout the cardiac cycle. Complex vortex dynamics, captured by PIV and CFD, persisted throughout the cardiac cycle on the central LAT plane. Velocity vector fields showed good overall agreement. For the BA, aneurysm agreement was more compelling, with both PIV and CFD similarly resolving the dynamics of counter-rotating vortices on both AP and LAT planes. Despite the imposition of periodic flow boundary conditions for the CFD simulations, cycle-to-cycle fluctuations were evident in the BA aneurysm simulations, which agreed well, in terms of both amplitudes and spatial distributions, with cycle-to-cycle fluctuations measured by PIV in the same geometry. The overall good agreement between PIV and CFD suggests that CFD can reliably predict the details of the intra-aneurysmal flow dynamics observed in anatomically realistic in vitro models. Nevertheless, given the various modeling assumptions, this does not prove that they are mimicking the actual in vivo hemodynamics, and so validations against in vivo data are encouraged whenever possible.

An objective approach to digital removal of saccular aneurysms: technique and applications.

Human studies of haemodynamic factors in the pathogenesis of cerebral aneurysms require knowledge of the pre-aneurysmal vasculature. This paper presents an objective and automated technique to digitally remove an aneurysm and reconstruct the parent artery, based on lumen geometries segmented from angiographic images. Relying on robust computational geometry concepts, notably Voronoi diagrams of the digitised lumen surface, the aneurysm attachment region is first defined objectively using lumen centrelines. Centrelines within this region are replaced by smooth interpolations, which then guide the interpolation of Voronoi points within the attachment region. Combined with Voronoi points from outside the attachment region, the parent artery lumen, without the aneurysm, can be reconstructed. Plausible reconstructions were obtained, automatically, for a set of 10 side-wall or terminal aneurysms, of various sizes and shapes, from the ANEURISK project data set. Application of image-based computational fluid dynamics analysis to a five side-wall aneurysm cases data set revealed an association between the recently proposed gradient oscillatory number (GON) and the site of aneurysm formation in four of five cases; however, elevated GON was also evident at non-aneurysmal sites. A potential application to the automated delineation of aneurysms for morphological characterisations is also suggested. The proposed approach may serve as a broad platform for investigating haemodynamic and morphological factors in aneurysm initiation, rupture and therapy in a way amenable to large-scale clinical studies or routine clinical use. Nevertheless, while the parent artery reconstructions are plausible, it remains to be proven that they are faithful representations of the pre-aneurysmal artery.

Is flow in the common carotid artery fully developed

The assumption of fully developed or axisymmetric velocity profiles in the common carotid artery (CCA) underlies the straightforward estimation of CCA blood flow rates or wall shear stresses (WSS) from limited velocity data, such as spectral peak velocities acquired using Doppler ultrasound. Using an automated velocity profile classifier developed for this study, we characterized the shape of the CCA velocity profile from cine phase contrast magnetic resonance images acquired as part of an Atherosclerosis Risk in Communities (ARIC) ancillary study, here focusing on 45 participants imaged twice as part of a repeatability protocol. When averaged over the cardiac cycle, roughly 60% of the velocity profiles were classified as skewed, with over half of these exhibiting the crescent shape characteristic of strong Dean-type flow in a curved tube. During early diastole, roughly 80% of the velocity profiles were skewed. In most cases the degree and orientation of skewing were reproduced in the repeat scan, indicating the persistence of these flow features. Fully developed flow thus appears to be the exception rather than the rule in the nominally straight CCA. Implications of this for flow rate and WSS estimation, and perhaps the development and progression of carotid atherosclerosis, warrant further investigation.

Hemodynamics of the Mouse Abdominal Aortic Aneurysm

The abdominal aortic aneurysm (AAA) is a significant cause of death and disability in the Western world and is the subject of many clinical and pathological studies. One of the most commonly used surrogates of the human AAA is the angiotensin II (Ang II) induced model used in mice. Despite the widespread use of this model, there is a lack of knowledge concerning its hemodynamics; this study was motivated by the desire to understand the fluid dynamic environment of the mouse AAA. Numerical simulations were performed using three subject-specific mouse models in flow conditions typical of the mouse. The numerical results from one model showed a shed vortex that correlated with measurements observed in vivo by Doppler ultrasound. The other models had smaller aneurysmal volumes and did not show vortex shedding, although a recirculation zone was formed in the aneurysm, in which a vortex could be observed, that elongated and remained attached to the wall throughout the systolic portion of the cardiac cycle. To link the hemodynamics with aneurysm progression, the remodeling that occurred between week one and week two of the Ang II infusion was quantified and compared with the hemodynamic wall parameters. The strongest correlation was found between the remodeled distance and the oscillatory shear index, which had a correlation coefficient greater than 0.7 for all three models. These results demonstrate that the hemodynamics of the mouse AAA are driven by a strong shear layer, which causes the formation of a recirculation zone in the aneurysm cavity during the systolic portion of the cardiac waveform. The recirculation zone results in areas of quiescent flow, which are correlated with the locations of the aneurysm remodeling.

Exploring High Frequency Temporal Fluctuations in the Terminal Aneurysm of the Basilar Bifurcation

Cerebral aneurysms are a common cause of death and disability. Of all the cardiovascular diseases, aneurysms are perhaps the most strongly linked with the local fluid mechanic environment. Aside from early in vivo clinical work that hinted at the possibility of high-frequency intra-aneurysmal velocity oscillations, flow in cerebral aneurysms is most often assumed to be laminar. This work investigates, through the use of numerical simulations, the potential for disturbed flow to exist in the terminal aneurysm of the basilar bifurcation. The nature of the disturbed flow is explored using a series of four idealized basilar tip models, and the results supported by four patient specific terminal basilar tip aneurysms. All four idealized models demonstrated instability in the inflow jet through high frequency fluctuations in the velocity and the pressure at approximately 120 Hz. The instability arises through a breakdown of the inflow jet, which begins to oscillate upon entering the aneurysm. The wall shear stress undergoes similar high-frequency oscillations in both magnitude and direction. The neck and dome regions of the aneurysm present 180 deg changes in the direction of the wall shear stress, due to the formation of small recirculation zones near the shear layer of the jet (at the frequency of the inflow jet oscillation) and the oscillation of the impingement zone on the dome of the aneurysm, respectively. Similar results were observed in the patient-specific models, which showed high frequency fluctuations at approximately 112 Hz in two of the four models and oscillations in the magnitude and direction of the wall shear stress. These results demonstrate that there is potential for disturbed laminar unsteady flow in the terminal aneurysm of the basilar bifurcation. The instabilities appear similar to the first instability mode of a free round jet.

Is Flow in the Common Carotid Artery Fully-Developed?

It is usually assumed, in both clinical and experimental settings, that blood velocity profiles in the common carotid artery (CCA) are fully-developed. This allows for a simpler estimation of CCA flow rates or wall shear stresses (WSS) from limited velocity data, such as peak velocities acquired using Doppler ultrasound (DUS) [1]. However, the assumption of a long, straight CCA may be incorrect, as the CCA does possess some curvature [2], which may alter the velocity profile from the assumed Poiseuille or Womersley shapes, and thus lead to incorrect inferences about flow rate or WSS. Consequently, this may have an impact on our understanding and diagnoses of cardiovascular related diseases. The aim of this study was to characterize the real shape of CCA velocity profiles in vivo.Copyright

Numerical Simulations of the Intra-Aneurysmal Vortex Shedding in Induced Mouse Abdominal Aortic Aneurysms

A feature of particular interest observed in vivo in murine abdominal aortic aneurysm (AAA) is the presence of a vortex shed from the proximal edge of the abdominal aortic aneurysm. It is unclear whether the occurrence of the shed vortex is due to the periodic nature of the flow-rate waveform, to geometric features, or to the compliant nature of the vessel tissue. Numerical simulations were performed on 3D semi-idealized and 2D axi-symmetric models of the abdominal aortic aneurysm at a mean Reynolds number of 63 and a Womersley number of 2 (for unsteady inflow conditions). The numerical results from the 3D model showed good agreement with the flows visualized by Doppler Ultrasound with respect to the development of the observed shed vortex. The idealized axi-symmetric models under steady flow conditions showed no signs of vortex shedding. Under unsteady inflow conditions, however, shear-layer roll-up occurred near the peak systolic velocity. The presence of a proximal lip was found to lead to vortex separation (from the wall) earlier in the cardiac cycle, and the presence of the proximal narrowing led to the earliest vortex separation. The sensitivity to the inflow waveform shape also showed that the presence of the shedding, in the model with proximal narrowing, disappeared when the peak-to-mean velocity ratio was reduced by approximately half. Therefore, we conclude that the observed intra-aneurysmal vortex shedding is a shear-layer rollup phenomenon; however, the geometry can act to enhance further the observed vortex shedding.© 2010 ASME

Image-Based CFD Modelling of Hemodynamic Factors in Aneurysm Formation Using a Novel Approach for Digital Removal of Saccular Aneurysms

Although local hemodynamic forces are widely believed to play a role in aneurysm pathogenesis, the hemodynamic mechanisms have not been confirmed in a prospective manner. Ideally, one would identify the patient-specific vessel that is prone to aneurysm formation and follow it longitudinally to investigate the associated aneurysm formation factors or mechanisms. However, such studies are not practical in humans, and so the knowledge to predict aneurysm formation at a specific location, a priori, is not available.Copyright

Hemodynamic Phenotypes of Anatomically Realistic Terminal Aneurysms

Intracranial aneurysms occur in approximately 4% of the population. While advances in medical imaging and surgical procedures have led to improved diagnosis and treatment, the decision of whether or not too treat an unruptured aneurysm is still largely subjective. The size of the aneurysm combined with its location and shape are the major determining factors, along with experience, when considering treatment. There is increasing recognition that hemodynamic forces play a key role in the life cycle of an aneurysm; however, it is difficult to provide this information in the clinic, owing to the need for time-consuming computational fluid dynamics (CFD) simulations. A more pragmatic solution, for now at least, may be to predict the gross flow patterns (“hemodynamic phenotype”) from simpler-to-measure geometric parameters.Copyright